1. Field of the Invention
This invention relates generally to control of a motor vehicle torque converter clutch during negative torque coast downs to avoid lash and clunk.
2. Description of the Prior Art
During an aggressive deceleration fuel shutoff (ADFSO) coasting event of a vehicle equipped with an automatic transmission, when its torque converter clutch transitions from hard lock, i.e., with zero slip, to a controlled negative slip, preferably about negative 40 rpm, unlocking the converter clutch can cause fuel supply to the engine to resume. This is primarily due to a “stick-slip” phenomenon wherein the dynamic coefficient of friction of the clutch in its slipping state differs from the static coefficient of friction in its locked state.
Due to the stick-slip occurrence, the torque transmitting capacity of the torque converter clutch must be substantially decreased to generate slip and then increased soon thereafter to maintain a controlled amount of negative slip. If the clutch actuating hydraulic pressure doesn't increase fast enough, then destroking the clutch causes the fuel injectors to pump fuel into the engine cylinders. If clutch actuating pressure increases too fast, then the clutch torque capacity is too large, reducing slip across the clutch causing a bump that can be felt by vehicle occupants. Destroking the clutch causes a loss of fuel economy and the related bump is attributed to harsh gear shifting of the transmission.
Different vehicles begin generating negative converter clutch slip at different clutch actuation pressures due to variability in transmission hardware, braking rate, and engine torque differences.
A need exists in the industry for a technique during a deceleration fuel shutoff coasting event to increase slip across the torque converter clutch before it decreases too far and to increase its torque capacity smoothly without causing destroke or bumps.